Discharge gap



. El?. KENNEDY DISCHARGE GAP Filed July 25, 1940 INVENTOR mi Kw4.

ATTORNEY Patented Dec. 9, 1941 DISCHARGE GAP Theodore R. Kennedy, Lower Makeiield Township, Bucks County, Pa., assignor to Ajax Electrotherrnic Corporation, Ajax Park, Ewing Township, Mercer County, N. J., a corporation of New Jersey Application July 25, 1940, Serial No. 347,357

7 Claims.

This invention deals with electric discharge gaps for converter equipments of the type used to supply high frequency induction furnaces.

An object of the invention is to provide a discharge gap of novel construction capable of operating at higher voltage and current ratings and with less electrical disturbance than those heretofore available.

A further purpose is to provide a discharge gap having sufficient mechanical strength to withstand the most violent explosion to which it is likely to be subjected without requiring vents or special safety devices, and without damaging the parts.

A further object is to provide a hermetically sealed discharge gap having suicient mechanical strength to withstand rough handling and any explosive force to which it is likely to be subjected without danger to the gap parts and without danger to an operator.

A further object is to provide a discharge gap in which the insulation between the high voltage parts is resistant to corona and progressive insulation failure.

A further object is to provide a discharge gap having an insulating stem with an external surface of fused silica and a ll of electrical insulating compound.

Further purposes appear in the speciiication and in the claims.

Two gures have been used to illustrate the invention. Figures 1 and 2 are, respectively, the plan and cross-sectional elevation views of one form of discharge gap embodying the improvements herein described.

Discharge gaps were quite widely used when radio was in its infancy and were developed somewhat in that art. However, interest in such gaps waned in the radio art when the vacuum tube carne into wide use and a really eflicient type of discharge gap for the lower range of converter frequencies was not developed until the advent of the converter type high frequency furnace. The furnace circuit or equipment diifered from that used by radio in that larger currents in general, had to be dealt with, efficiency was of more importance and the discharge gap had to be silenced and housed so that an operator could Work around it with safety and without discomfort.

It was early learned that a converter discharge gap wherein the electric discharge passed from a solid metallic electrode to a pool of mercury, through an atmosphere of a hydrocarbon or hydrogen gas was an enicient combination,

and U. S. Patent 1,594,846 was granted to E. F. Northrup August 3, 1926 covering this combination.

In discharge gaps of the type herein described the Aservice is severe. The voltages normally handled are from 5000 to 10,000 volts With peaks or transients estimated in excess of 20,000 to 30,000 volts. Sustained current loads in excess of 250 amperes must be provided for.

The heat of the arc striking the mercury causes it to vaporize and to form a more or less conducting path throughout the discharge gap. This vapor, together with the heat radiated or conducted from the arc, and together with other vaporized particles from the solid electrode tip, puts an unusual and excessive strain on the insulation between the parts. In addition to this a combustible gas atmosphere is used which subjects the discharge gap to frequent explosions. These explosions can be and have often been violent and damaging.

In most discharge gaps the length of the gap proper must be varied as a means of varying the power and since a gas proof seal must be used this brings in a still further diiliculty.

Because of insulation difliculties, the unstable or erratic operation due to voltage spill-overs, the fear of explosion and other general obstacles the converter type high frequency furnace equipment has been limited to comparatively small scale work or to jobs where an intermittent operation could be tolerated. It is significant that no such commercially available equipment had been .built prior to this invention in excess of a 20 kw. rating. This has been most disconcerting to the industry as there is much Work t0 which the type of Wave form and frequency of the converter type furnace can be applied more effectively than frequencies obtained from rotary generating equipment, and rotary generating equipment has not been available in the range of frequency and power most desired.

The present invention has been effective in increasing the range of power obtainable from converter equipment and it is believed Will so increase the stability of operation of this type of equipment that it can be used on a large variety of work for which heretofore no practical source of power has been available.

The major improvement in the new discharge gap is that the container or housing has been built of steel or other suitable material having a substantial Wall thickness and a mechanical strength suiiicient to withstand danger from rupture due to explosion. All parts of the discharge gap housing are mechanically strong and are effectively sealed against movement. So far as applicant can ascertain this is the rst time that such a discharge gap has been made withoue having some part which could break or give way to offset the danger of explosion. In all previous assemblies the electrodes or insulating sleeves have been slidably removable, or parts of the gap have been made of rlbre or thin glass, or mica blowout windows have been installed, or some provisicn has been made to vent an eXplosive force with the least mechanical disturbance. The discharge gap chambers have had to be so shielded that such ruptures as have occurred would not be d-angerous to persons standing near-by.

It appears that in all previous development discharge gaps of the present design have been scrupulously avoided. The idea that such a discharge gap might be made explosion proof apparently had not occurred to others.

After much study and experimentation applicant learned that any explosion which might occur could be mechanically held within the discharge gap Vchamber without resorting to eX- ploson venting devices. Other improvements were combined to make a substantially new type of discharge gap.

Discharge gap failures often are caused by corona leakage or progressive insulation failure across or through the discharge gap insulation. Where the outside wall is of insulating material a deposit usually collects thereon which in time is suiiiciently conducting to invite a corona or brush discharge, followed by a voltage spill-over orl'breakdown. If the insulating surface is cold, mercury vapor condenses thereon facilitating an initial discharge. Where the insulation is a sleeve around the solid electrode the corona leakage or brush discharge usually penetrates the sleeve at the point of greatest strain and a breakdown follows. Failure is also enhanced by the high frequency.

In the present discharge gap applicant has used the internal sleeve type insulator. This permits the whole outer wall to be of metal, allowingV for a maximum of cooling surface. Because of the smaller insulating sleeve required this can be made effectively of smooth fused silica. Applicant has found a silica sleeve to be particularly effective in avoiding ultimate breakdowndue to corona or progressive insulation failure. to advantage.

To eliminate corona or progressive insulation failure between the solid electrode and theV steel housing through the insulating sleeve and to maintain a desired temperature on the insulation wall Vsurface a plastic insulating compound is used in the space between the insulating sleeve and the electrode. This compound being liquid on assembly lls all voids. In operation it is soft and pliable and has a low melting point. When a failure tends to occur the portion of the compound adjacent to the point of incipient breakdown melts and reseals the insulation. Even if the insulating sleeve cracks the discharge gap still is operative as the plastic compound is maintained sufficiently solid by the cooling in the electrode so that it will not run out into the discharge gap chamber.

Applicant sometimes uses this same insulating compound over the top external surface of the discharge gap to prevent the formation of corona or breakdown at that point. I-Ie believes the Other materialsv may of course be usedY reason this type of insulation is better than other insulators is that moisture condenses thereon in minute spheroids rather` than as a lm.

Applicant has so designed the new discharge gap that all insulating parts are in compression and he has used resilient packings and overflow vents to prevent rupture due to thermal expansion of the insulating compound.

Discharge gaps of the type herein described can be made either of the hermetically sealed. or of the hydrogen flow type. When making the hermetically sealed type they lend themselves particularly well to sealing off operations at pressures above or below atmospheric.

While a mercury hydrogen discharge gap has been herein described it is not the intention of applicant to restrict his invention to this type. While a mercury electrode is of great value two solid electrodes may be used. The invention may also be applied to other types.

Referring to the figures, which are illustrative rather than limiting of the invention, a discharge gap is shown which comprises a solid electrode I, spaced from a liquid electrode 2, housed in a double walled metallic container 3, 4. The outer container 4 is adapted with ports 5 for the circulation of a cooling fluid, and the inner container is or may be hermetically sealed by gaskets i5, l, 8, insulating sleeve 9, preferably of fused silica, and an insulating compound I0. A sealing off tube II may be used if a gas is to be sealed within the inner chamber, or two such tubes, appropriately placed, may be used if a gas is to be passed through the inner chamber.

The center electrode is arranged for cooling as in usual practice.

On assembly sealing gaskets 6 and 1, expansion packing I2, and the insulating sleeve 9 are dropped over the solid electrode shank. The space between the sleeve and electrode is lled with molten insulating compound and the insulating top I3 is screwed down over threads on the upper end of the solid electrode to engage the gaskets and to put the insulating sleeve into compression. During this operation excess compound flows through ports I 4 and past gasket T. The insulating collar I5 serves as a baffle to displace some of the compound on assembly and to restrict the compound to a narrow band around the electrode in use. The expansion packing I2 serves to take up variations in volume due to expansion and contraction of the insulating compound in use. f

After assembly of the solid electrode system, the gasket 8 is placed and the top I3 is rigidly fastened by bolts I6 to the upper ring I'I of the metallic housing. Sealing compound is poured over the top piece I3 to complete the seal, and

Va Bakelite or other cover I8, is placed and held in position by nuts I. The sealing compound may have a wide range of plasticity, even including that of the usual insulating oils although the semi-plastic compounds are preferred.

The whole discharge gap is made of materials sufficiently strong to withstand the most violent explosion which may occur between a mixture of air and hydrogen or other` gas contained within the gap chamber, under usual conditions. A maximum pressure of 200 pounds per square inch usually is provided for. l

Electrical contacts are made by a stud or connection 20 fastened to the metallic portion of thevchamber 4 and by a connection 2|, held in place between the nuts I9 at the top of the electrode.

Trunnions 22 may be fastened to the discharge gap sides to facilitate tilting to vary the gap length, or a separate reservoir and plunger or leveling system not shown, may be used.

Having thus described his invention, what applicant claims as new and desires to secure by United States Letters Patent is:

1. A discharge gap having walls surrounding electrodes and enclosing a combustible gas of the order of hydrogen, said walls being adapted to withstand without volumetric change of the chamber enclosed an internal pressure approximating 200 pounds per square inch and any pressure which may be developed by iniiltration of air and combustion of said gas.

2. A discharge gap having relatively spaced electrodes insulated from one another, walls surrounding the electrodes and forming a substantially totally closed system, a combustible gas within said walls capable on combination with air and ignition of creating an internal pressure upwards of 200 pounds per square inch and comparable to that produced by air hydrogen mixtures at atmospheric pressure, said walls being adapted to withstand said pressures without venting means.

3. A discharge gap having a solid electrode, a mercury electrode spaced and insulated from said solid electrode, walls surrounding the electrodes and forming a substantially totally closed system, a combustible gas within said walls capable on combination with air and ignition of creating an internal pressure upwards of 200 pounds per square inch and comparable to that produced by air hydrogen mixtures at atmospheric pressure, said walls being adapted to withstand said pressures without venting means.

4. A discharge gap having relatively spaced electrodes said electrodes being insulated from one another and sealed with a combustible gas atmosphere within walls which, together with the sealing mechanism, form an explosion proof chamber capable of withstanding internal pressures of the order of 200 pounds per square inch, without venting means, said insulation comprising in part a solid dielectric and a dielectric which is solid at normal operating temperature but which liquiiies at slightly higher temperature.

5. A discharge gap having relatively spaced electrodes said electrodes being insulated from one another and sealed with a combustible gas atmosphere within Walls which, together with the sealing mechanism, iorm an explosion proof chamber capable of withstanding internal pressures of the order of 200 pounds per square inch, without venting means, said insulation comprising in part a solid dielectric and a dielectric which is gas tight and electrically self-healing.

6. A discharge gap comprising a water cooled solid electrode, a mercury electrode, a water cooled tank enclosing said mercury electrode, said solid electrode projecting into but insulated from said tank, an insulating sleeve surrounding a portion of the shank of said solid electrode and forming in part the insulation between it and the tank walls and a lill of an electrical insulating compound between said sleeve and said electrode, said tank assembly being capable of withstanding internal pressures of the order of 200 pounds per square inch without venting means.

7. A discharge gap comprising a water cooled solid electrode, a mercury electrode, a water cooled tank enclosing said mercury electrode, said solid electrode projecting into but insulated from said tank, an insulating sleeve surrounding a portion of lthe shank of said solid electrode and forming in part the insulation between it and the tank walls, a fill of electrical insulating compound between said sleeve and said shank affording additional and self-healing insulation between said parts and at the same time making a gas tight seal for the electrode chamber, the construction as a whole rendering the discharge gap explosion proof against internal pressures approximating 200 pounds per square inch.

THEODORE R. KENNEDY. 

